Non-invasive blood pressure measurement is often done by measuring the pulse wave transit time from the heart to the finger. Namely the pulse wave transit time and thus the pulse wave velocity are dependent on the blood pressure. Also, changes in blood pressure can be measured by measuring changes in pulse wave transit time or pulse wave velocity (PWV). The prior art methods are measuring PWV continously from the electrocardiogram (ECG) r-wave to the pulse oximeter plethysmogram wave crest. These measurements require calibrating the value with a standard non-invasive blood pressure cuff reading. A typical measurement method of this kind is oscillometric cuff measurement.
Another prior art measurement method is a standard finger oximetry pleth measurement.
The prior art measurement principles have problems at both ends; the delay from the heart electrical-to-mechanical activity is variable and not easily controllable, and the standard finger site for oximetry pleth is very sensitive to vasoconstriction, that affects the hand, i.e. palm and finger, pulse delays in a highly variable manner. Often, as with sick patients with low peripheral perfusion, the finger pulse is not detectable at all or very noisy.
The heart side of the measurement problem has in prior art been solved by adding an ear plethysmographic probe, and monitoring the ear-finger pulse transit time; the obvious drawback is the extra sensor needed. In practical clinical conditions the extra sensor is difficult to use.
Circulation and Blood Pressure
In this invention, three physiological signals originating from the circulatory system are measured to produce continuous information on blood pressure changes: the electrocardiogram, the impedance cardiogram, and the photoplethysmogram arising from a pressure pulse passing through a vessel. In addition, an intermittent blood pressure measurement method is used for repetitive calibration.
Circulatory System
FIG. 1 shows the circulatory system of a person. The system consists of both the systemic 101 and the pulmonary 102 circulation. The circulatory system of a person or patient also consists also of the heart 103. The pulmonary circulation 102 supplies the lungs 104 with blood flow, while the systemic circulation takes care of all the other parts of the body i.e. the systemic circulation 101. The heart 103 serves as a pump that keeps up the circulation of the blood. The systemic circulation consist of the venule 106, the capillary system 108 and the arteriole 109.
Blood Pressure
Blood pressure is defined as the force exerted by the blood against any unit area of the vessel wall. The measurement unit of blood pressure is mmHg. This means millimeters of mercury.
FIG. 2 shows the definition of systolic 202 and diastolic 204 blood pressure values. Pulmonary and systemic arterial pressures are pulsatile, having systolic 202 and diastolic 204 values. As illustrated in FIG. 2, the highest recorded pressure reading is called systolic pressure 202. It results when the volume of the heart decreases during contraction, while the volume of blood in the circulation remains constant. The lowest pressure reading is called diastolic pressure 204.
Electrical Activity of the Heart
The pumping action of the heart is a consequence of periodical electrical events occurring in the cardiac muscle tissue. These electrical events can be measured by an electrocardiogram ECG and they are further elaborated in the following in connection with FIG. 3.
Events of the Cardiac Cycle
FIG. 3 shows the timing of the electrical and mechanical events during one cardiac cycle. A cardiac cycle consists of all the events that occur between the beginning of a heartbeat and the beginning of the next heartbeat.
The P wave 305 of the ECG curve 303 is caused by the depolarization of the atria. It is followed by atrial contraction, indicated by a slight rise in the atrial pressure. The QRS wave 307 of the ECG 303 appears as the ventricles depolarise, initiating the contraction of the ventricles.
The repolarisation 309 of the ventricles, indicated by the T wave of the ECG, suddenly causes the ventricles to begin to relax.
Pulse Wave Transit Time Method
Many experiments are reported in which pulse wave velocities or pulse transit times were measured and used to evaluate blood pressure or blood pressure change.
In practice, pulse wave transit times are usually measured rather than velocities. Peripheral pressure pulses are detected by photoplethysmography.
The current opinion is that changes in pulse wave velocity or transit time indeed predict blood pressure changes.
In this text, pulse wave transit time is the time that elapses as a pulse wave propagates from one site to another. It is inversely proportional to the velocity of the pulse wave. A delay is the time between two events. It may include propagation periods and other time lapses.
Measurement Principle
When the left ventricle of the heart contracts and ejects blood into the aorta, only the proximal portion of the aorta becomes distended. The distension then spreads as a wave front along the walls of the arteries and arterioles. The velocity of the pulse wave is 3–5 m/s in the aorta, 7–10 m/s in large arterial branches, and 15–35 m/s in small arteries. In general, the smaller the distensibility of the vessel wall, the faster the pulse wave propagates. The total transit time from the aortic root to the periphery is in the order of 100 ms. FIG. 3 gives an example of four pressure waveforms measured at different sites after the ejection of blood from the heart.
As the pulse propagates towards the periphery, the vessel diameter and the distensibility of the vessel wall decrease, changing the transmission properties and distorting the pulse contour. Most of the distortion is, however, caused by reflected pulse waves that combine with the pulses travelling towards the periphery. The main reflection occurs as the pulse wave reaches the high-resistance peripheral arteries, but arterial lesions or junctions of large arteries can cause additional reflections.
Consider two cardiogenic signals that can be obtained with surface electrodes: the electrocardiogram (ECG) and the impedance cardiogram (ICG). Table 1 summarizes their characteristics and suitability for timing the onset of the pressure pulse.
TABLE 1Suitability of two different signals fordetermining the onset time of the pulse wave.signalECGICGwhat is meas-electrical poten-cardiac related impedanceuredtials on the surf-changes by applying currentace of the thoraxand measuring potentials onthe surface of the thoraxorigin of theelectrical acti-mechanical function ofsignalvation of the heartthe heartadvantagesR waves are readilyindicates the true onsetdetectabletime of the pressure pulsedrawbacksthe PEP between thethe signal may be noisyR wave and the onsetand formless, and theof the pressure pulseactual origin is obscuremay not be constant(thus the question mark)Possible Problems
There are numerous physiological factors that influence the pulse wave velocity and/or the pre-ejection period (PEP). Most of these unpredictable mechanisms act on the PEP, not on the actual transit time. Elimination of the contribution of the PEP was thus supposed to improve the technique.
A more likely problem associated with the sympathetic mechanisms is the vasoconstriction of the peripheral arteries, triggered by emotional stress, cold, exercise, or shock. Vasoconstriction changes the peripheral resistance, thus affecting pulse wave transmission and reflection.
Electrocardiography
Electrocardiogram (ECG) is a recording of electrical potentials generated by the function of the heart. The ECG is measured as potential differences between electrodes placed on the surface of the body on standardized positions. This is depicted especially in connection with FIG. 4.